Breakdown of dynamic scaling in surface growth under shadowing

Growth front smoothing effects in extremely high pressure vapor deposition

Recent experimental chemical vapor depositions of silicon at extreme pressures of ~ 50 MPa (~ 500 atm) have been observed to generate remarkably smooth surfaces not predicted by low-pressure deposition models. In this paper, we propose an anti-shadowing mechanism where the collision of particles within the valleys of the surface growth front leads to smoothening. We conduct Monte Carlo simulations to simulate the evolution of film roughness at pressures between 1 and 50 MPa. We observe that surface roughness approaches an asymptotic invariant value that follows power law behavior as a function of pressure. The film thickness at which invariance begins is shown to have a similar power law behavior with respect to pressure. Our simulated results compare favorably with recent experimental observations and provide insight into the fundamental mechanisms underlying film evolution at pressures between one and hundreds of atmospheres.

Chemical Vapor Deposition of Carbon Nanocoils Three-Dimensionally in Carbon Fiber Cloth for All-Carbon Supercapacitors

An Au/K bicatalyst-assisted chemical vapor deposition process using C₂H_2(g) to grow high-density carbon nanocoils (CNCs) uniformly on the fibers in carbon fiber cloth substrates three-dimensionally was developed. An as-deposited substrate (2.5 × 1.0 cm²) showed a high electrochemical active surface area (16.53 cm²), suggesting its potential usefulness as the electrode in electrochemical devices. The unique one-dimensional (1D) helical structure of the CNCs shortened the diffusion pathways of the ions in the electrolyte and generated efficient electron conduction routes so that the observed serial resistance R _s was low (3.7 Ω). By employing two-electrode systems, a liquid-state supercapacitor (SC) in H₂SO_4(aq) (1.0 M) and a solid-state SC with a polypropylene (PP) separator immersed in H₂SO_4(aq) (1.0 M)/polyvinylalcohol were assembled and investigated by using CNC-based electrodes. Both devices exhibited approximate rectangular shape profiles in the cyclic voltammetry measurements at various scan rates. The observations indicated their electric double-layer capacitive behaviors. From their galvanostatic charge/discharge curves, the specific capacitances of the liquid SC and the solid SC were measured to be approximately 137 and 163 F/g, respectively. In addition, the solid-state CNC-based SC possessed excellent energy density (15.3 W h/kg) and power density (510 W/kg). The light weight solid SC (0.1965 g, 2.5 × 1.0 cm²) was bendable up to 150° with most of the properties retained.

Large-area zinc oxide nanorod arrays templated by nanoimprint lithography: control of morphologies and optical properties

Vertically aligned, highly ordered, large area arrays of nanostructures are important building blocks for multifunctional devices. Here, ZnO nanorod arrays are selectively synthesized on Si substrates by a solution method within patterns created by nanoimprint lithography. The growth modes of two dimensional nucleation-driven wedding cakes and screw dislocation-driven spirals are inferred to determine the top end morphologies of the nanorods. Sub-bandgap photoluminescence of the nanorods is greatly enhanced by the manipulation of the hydrogen donors via a post-growth thermal treatment. Lasing behavior is facilitated in the nanorods with faceted top ends formed from wedding cakes growth mode. This work demonstrates the control of morphologies of oxide nanostructures in a large scale and the optimization of the optical performance.

Nanostructured surfaces investigated by quantitative morphological studies

The morphology of different surfaces has been investigated by atomic force microscopy and quantitatively analyzed in this paper. Two different tools have been employed to this scope: the analysis of the height-height correlation function and the determination of the mean grain size, which have been combined to obtain a complete characterization of the surfaces. Different materials have been analyzed: SiO(x)N(y), InGaN/GaN quantum wells and Si nanowires, grown with different techniques. Notwithstanding the presence of grain-like structures on all the samples analyzed, they present very diverse surface design, underlying that this procedure can be of general use. Our results show that the quantitative analysis of nanostructured surfaces allows us to obtain interesting information, such as grain clustering, from the comparison of the lateral correlation length and the grain size.

Static and dynamic aspects of black silicon formation

We present a combination of experimental data and modeling that explains some of the important characteristics of black silicon (BSi) developed in cryogenic reactive ion etching (RIE) processes, including static properties (dependence of resulting topography on process parameters) and dynamic aspects (evolution of topography with process time). We generate a phase diagram predicting the RIE parameter combinations giving rise to different BSi geometries and show that the topographic details of BSi explain the metamaterial characteristics that are responsible for its low reflectivity. In particular, the unique combination of needle and hole features of various heights and depths, which is captured by our model and confirmed by focused ion beam nanotomography, creates a uniquely smooth transition in refractive index. The model also correctly describes dynamical characteristics, such as the dependence of aspect ratio on process time, and the prediction of new etching fronts appearing at topographical saddle points during the incipient stages of BSi development--a phenomenon reported here for the first time.

Influence of microstructure on the transitions between mesoscopic thin-film morphologies in ballistic-diffusive models

We study the influence of the symmetries of competing microstructures on the emergence of different mesoscopic morphologies in the growth by vapor deposition of thin solid films. We show the results of numerical simulations in (1+1) - and (2+1) -dimensional systems including different microstructures, as well as thermally activated surface diffusion in combination with a ballistic algorithm to model the deposition process. We focus on the characterization of the transitional structures that appear in the empirical structure zone model (SZM) through the evaluation of the mean packing density and the mean coordination number. We show that the maximum coordination number of the underlying microstructure classifies the statistics of the transitional morphologies at the border between zone I in the SZM, characterized by the formation of fractal-like patterns, and zone II, where pronounced faceting develops. We analyze the appearance of lattice frustration and texture competition effects in complex microstructures having mutually exclusive symmetries.

Unified moving-boundary model with fluctuations for unstable diffusive growth

We study a moving-boundary model of nonconserved interface growth that implements the interplay between diffusive matter transport and aggregation kinetics at the interface. Conspicuous examples are found in thin-film production by chemical vapor deposition and electrochemical deposition. The model also incorporates noise terms that account for fluctuations in the diffusive and attachment processes. A small-slope approximation allows us to derive effective interface evolution equations (IEEs) in which parameters are related to those of the full moving-boundary problem. In particular, the form of the linear dispersion relation of the IEE changes drastically for slow or for instantaneous attachment kinetics. In the former case the IEE takes the form of the well-known (noisy) Kuramoto-Sivashinsky equation, showing a morphological instability at short times that evolves into kinetic roughening of the Kardar-Parisi-Zhang (KPZ) class. In the instantaneous kinetics limit, the IEE combines the Mullins-Sekerka linear dispersion relation with a KPZ nonlinearity, and we provide a numerical study of the ensuing dynamics. In all cases, the long preasymptotic transients can account for the experimental difficulties in observing KPZ scaling. We also compare our results with relevant data from experiments and discrete models.